Many suction gripper devices are known in the prior art, comprising a suction plate having at its underside surface, suction openings that can be sealed by calibrated flaps and are connected to low pressure means. Such devices have been the object of many improvements in order to facilitate the holding of objects distorted by twisting, or featuring splits or irregular edges. The inner surface of the plate generally comprises a plurality of openings, from several tens to several hundreds of them, to allow suction and therefore to allow the opposite object to lie flat on it. In order to prevent excess air from entering through the opening covered by the flat object to be lifted, each opening is provided with a calibrated flap consisting of a vertically freely moving stem topped by one or more rubber or metal washers. The weight of such washers is adjusted so that flaps remain open when the air flow-rate through the respective opening is small and even non-existent, which is the case when an object lies flat against said opening. By contrast, when no object lies flat against given opening, the flap is sucked upwards and obturates the relevant opening.
In the case of large plates, a very large instantaneous flow-rate is required to lift all the flaps associated with the suction openings that are not obturated by the object to be lifted. More particularly, when the board is not large, it covers only a small portion of the openings and the flow-rate must be large enough to lift all the other flaps. To achieve this large instantaneous flow-rate, one solution is to provide a high-capacity vacuum pump. However, this results in having an oversized pump and it has been suggested in the prior art to insert a vacuum vessel betwen the pump and the suction plate. This vacuum vessel, through a closable conduit, is connected with the chamber delimited by the suction plate. This chamber delimited by a suction plate, through a second closable opening, is also connected with ambient air. The sequence of operations is as follows: first the vacuum opening and the air connection opening are closed. Therefore, the pump, creates a vacuum within the low-pressure vessel, of relatively large magnitude. When the suction plate is positioned above the object to be lifted, the sealing opening opens, which creates a strong draught within the chamber delimited by the suction plate, said draught being large enough to hold all flaps associated with the openings not covered by the object to be lifted lying flat in the sealing position.
During the next phase, the object is kept laying flat against the suction plate and a limited flow-rate is sufficient since all the openings of the underside surface of the plate are obturated by the flaps or the object to be lifted. The sole purpose of the air flow-rate required to maintain an adequate vacuum value is to make up for eventual leakages.
The third phase is the laying down of the object that has been held flat against the suction plate This is achieved by opening the conduit for connection with ambient air, which eliminates the vacuum inside the chamber delimited by the suction plate. This also eliminates the vacuum inside the vacuum vessel.
Obviously it could be considered to close the vacuum opening simultaneously or previously. However, this would be most detrimental to the longevity of the pump which would in this case work in this portion of its efficiency curve associated with low flow-rate and very large vacuum, that is, at limit capacity. For this portion of its efficiency curve, the pump requires a high power and its energy consumption is high. Also, an oversized motor should be required to prevent detrimental overheating both of motor and of mechanical components.
By contrast, it is possible to escape these disadvantages by leaving open the vacuum opening. But unfortunately, much time, several seconds in fact, is needed to rebuild vacuum, from the instant that the atmosphere connection opening is closed again to the instant when the vacuum prevailing inside the low-pressure vessel is again adequate to keep all flaps lying flat in their sealing position. To keep this rebuild time as short as possible, it is necessary to reduce volume, which cuts down the number of flaps and gives rise to safety problems (smaller reserve in case of power failure). Moreover, if the atmosphere connection opening is closed early and if the vacuum opening is not open when vacuum reaches the operating level, the above-mentioned problems are met again.